Process for forming crystalline polymer pellets

ABSTRACT

This invention relates to a process and apparatus for forming a polyester polymer into particles. More particularly, this invention relates to a process and apparatus for forming crystalline, uniform pellets from an amorphous polyester melt. The polyester pellets have utility, for example, as feedstock for a process for producing higher molecular weight polyesters.

This is a division of application Ser. No. 08/376,599, filed Jan. 20,1995, now U.S. Pat. No. 5,633,018.

FIELD OF THE INVENTION

This invention relates to a process and apparatus for forming a polymerinto particles. More particularly, this invention relates to a processand apparatus for forming crystalline, uniform pellets from an amorphouspolyester melt.

BACKGROUND

The formation of particles from viscous materials is well known.Conventional methods and apparatus often involve the formation of liquidportions or droplets which are subsequently collected and solidified.For example, Froeschke, U.S. Pat. No. 4,279,579, discloses an apparatusfor the extrusion of a flowable mass onto a conveyor. The apparatus hasinner and outer cylindrical coaxial containers. The inner container,positioned within the inner container, has a passage for dispensing theflowable mass. The outer container has a number of orifices and rotatesaround the inner container. As the outer container rotates, the orificeson the outer container cyclically align with the passageway on the innercontainer. With each alignment, the flowable mass flows from the innercontainer, through the aligned orifices, and is apportioned anddeposited on a conveyor, for example a conveyor belt, to form what isoften referred to as pastilles.

Chang et al., U.S. Pat. No. 5,340,509, discloses a pastillation processfor pelletizing ultra high melt flow crystalline polymers, i.e., acrystalline polymer which is a polyolefin homopolymer, a polyolefincopolymer, or blends thereof. Initially, molten polymer is transferredto a droplet-forming means. The droplet-forming means is generally anouter container, with orifices, which rotates around an inner containerto allow a uniform amount of the polymer melt to emerge as droplets. Thedroplets are collected on a conveyor, which cools the droplets for atime sufficient to solidify the droplets.

Forming robust, uniform, pellets of a polyester material has beendifficult or problematic. For example, low molecular weight polyesters,characterized as oligomers or prepolymers, may have such a low viscositythat initial particle formation may be difficult. The oligomer may betoo liquid to form particles or pellets of uniform shape and size. Thisis because oligomers, having relatively short chain length, may have arelatively low amount of chain entanglement, in addition to limitedintermolecular bonding or forces.

Known processes for forming polyester particles may result in particleswhich lack structural integrity. The weakness of such particles may makethem hard to handle and susceptible to abrasion during transport orother mechanical handling. Abrasion may generate undesirable amounts offines.

Polyester particles are useful as feedstock to a process to produce ahigher molecular weight polymer, including solid-phase ("solid-state")polymerization processes. For such processes, it is desirable that theparticles have certain characteristics. For example, particles havingrelatively uniform size and shape, for uniform polymerization withineach particle, may be desirable. For solid-state polymerization, it isdesirable that the particles be sufficiently robust to withstand thehigh temperatures of solid-state polymerization without agglomerating.

Conventionally, robust particles of polyester may be obtained bysubjecting the particles to a lengthy and expensive heat treatment orannealing step. Such annealing increases the crystallinity androbustness of the particles. Such annealing, however, typically addstime and expense to an overall process for producing high molecularweight product. It would be desirable to reduce or eliminate suchannealing.

In view of the above, there exists a need for an improved process andapparatus for the formation of polyester particles. There is a need forthe more economical and efficient production of quality polyesterparticles, which, for example, are useful under rigorous circumstancesand with limited pre-treatment prior to use as feedstock for furtherpolymerization. Furthermore, there exists a need for an improved processof forming a low molecular weight polyester oligomer into crystallineparticles. In addition, it would be a further advantage if the resultingparticles exhibited improved crystalline morphology or relatedproperties compared to conventional processes.

SUMMARY OF THE INVENTION

This invention provides an apparatus for producing pellets of a polymerfrom its polymer melt, comprising:

(a) a pellet former comprising a rotatable container having a pluralityof outlets, defining corresponding openings 0.5 to 5 mm in diameter, formetering a polymer melt onto the surface of a conveyor;

(b) a conveyor comprising a surface, which is adapted for movementrelative to the outlets of the pellet former, for receiving the polymermelt, from the pellet former, in the form of a plurality of droplets orcrystallizing pellets, said conveyor being adapted for conveying thepellets through a crystallization section; and

(c) a crystallization section extending from the point at which thepellets are received onto the surface of the conveyor, extending alongat least a portion of the conveyor to a point downstream;

the crystallization section further comprising means for controlling thesurface temperature of the conveyor within a predetermined temperaturerange above 50° C. as the surface passes through the crystallizationsection.

In commercial practice, the crystallization section may further comprisea temperature controller for controlling the temperature of the surfacewithin the crystallization section, such that the pellets are subjectedto a surface within a predetermined temperature range for apredetermined period of time.

The above-described apparatus may have a variety of uses, including theproduction of pellets of a polyester polymer having a glass transitiontemperature (T_(g)) greater than about 25° C. One such processcomprises:

(a) metering a polymer melt of the polyester polymer through a multitudeof outlets in a rotatable container, each outlet defining an orifice 0.5to 5 mm in diameter, thereby forming a plurality of molten droplets;

(b) collecting the molten droplets, immediately after being formed, on asolid moving surface, the solid moving surface being maintained within apredetermined temperature range within a heating zone, whereby thepellets are maintained in contact with the solid moving surface, withinthe heating zone, for a predetermined period of time.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of the preferred process and apparatus forproducing polymer pellets.

FIG. 2 is a cross-sectional view of the crystallization section of theapparatus of FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

This invention provides an apparatus and process for producing lowmolecular weight polymer particles or pellets. The polymer pellets areproduced in a pellet former commonly referred to as a pastillator, andare collected on a hot surface. The hot surface controls the rate atwhich the pellets are relatively cooled (from the melt) and thetemperature to which the pellets are relatively cooled. The pellets thusformed may have a relatively uniform size and shape. By the term"relatively uniform" is meant that at least 90 percent, by weight, ofthe pellets are within plus/minus 30 percent of the mean diameter.Preferably, at least 95 percent, by weight of the particles are withinplus/minus 10 percent of the mean diameter.

The present process is capable of producing pellets that are strongerand more abrasion resistant than pellets formed by various otherconventional methods and apparatus. The pellets are suitable fortransport or subsequent treatment by solid-state polymerization, with orwithout additional annealing.

One embodiment of the present invention, including an apparatus forproducing pellets, is shown schematically in FIGS. 1 and 2.

For purposes of this invention, the term "pellet" means any discreteunit or portion of a given material, having any shape or configuration,whether regular or irregular. Thus, the term "pellet" may encompassparticles, droplets, pieces, portions, or pastilles of a given material.By the term "polymer" is meant a compound or mixture of compoundsconsisting essentially of repeating structural units called monomers,and is meant to include a prepolymer or a oligomer, that is, a polymerhaving a low molecular weight or a polymer intended as feedstock for ahigher molecular weight polymer.

By the term "molten polymer" is meant polymer at a temperature at orabove its melt temperature. Likewise, by the term "molten droplet" or"droplet" is meant a portion of a polymer at least partially attemperature at or above the melting point of the polymer. Thus,temperature gradients may exist in the droplet, which may startcrystallizing immediately after being formed.

The melting point (T_(m)) of a polymer is preferably determined as themaximum of the main melting endotherm on the first heat, measured byDifferential Scanning Calorimetry (DSC). By pellet size is meant thelargest cross-sectional dimension of a given pellet.

As part of art integrated process, the droplet former may be incommunication, via a conduit or other material transfer means, with ameans for producing a polymer in melt form. A means for producing apolymer melt can encompass many variations. For example, the means canbe an extruder which uses polymer in the form of flake, pellets or chipsas a feedstock. An extruder can heat the feedstock to the melttemperature or higher and extrude the molten polymer in various shapes,for subsequent transfer to the droplet former.

The means for producing the polymer can also include a reactor forpolymerization. Such a reactor is well known in the art. Polymerizationis often carried out in the melt, and thus melt polymerizers are alsosuitable as a means for producing the polymer in melt form per thisinvention. An example of a preferred reactor for producing polymer meltis described in copending commonly-assigned application Ser. No.08/376,596, now abandoned and refiled as CIP Ser. No. 08/576,657, filedDec. 21, 1995, hereby incorporated by reference in its entirety. Ofcourse, for use as feedstock for the present apparatus and process, itis also possible to commercially purchase polymer or to store previouslymade polymer for later introduction into a means for producing a melt ofthe polymer.

One preferred embodiment of the present apparatus is schematically shownin FIG. 1. A pellet former 10 receives a polymer melt from a reactor ormelt polymerizer (not shown). A conventional melt polymerizer, ifemployed, usually has an inlet for receiving reactants and an outletconnected to a conduit for transporting the polymer melt to the pelletformer 10. The polymer exiting the outlet is typically at or above itsmelt temperature. The polymer can be transferred to a pellet former bymeans of any pressure displacing device such as a variable speeddisplacement pump or melt gear pump.

The pellet former 10 is commonly referred to as a pastillation apparatusor pastillator, in the broadest sense of the word. Various types ofpastillators are known in the art for various uses. The pastillator, inone embodiment, may typically comprise inner and outer coaxialcylindrical containers. Accordingly, polymer melt transferred from thereactor would be received into the inner container or cylinder. Theouter container has a plurality of orifices circumferentially spaced onthe periphery of the outer container. The plurality of orifices aredisposed such that they align with a metering bar or channel on theinner container when the outer cylinder is rotated. The orifices on theouter container can typically range in size from about 0.5 mm to about 5mm. The inner cylinder containing the polymer melt is under pressure anddispenses the melt in uniform amounts as each of the plurality oforifices on the outer cylinder align with the metering bar or channel onthe inner cylinder. Pastillators, as described, are commerciallyavailable, e.g., ROTOFORMER® manufactured by Sandvik Process Systems(Totawa, N.J.). In commercial use, for economic efficiencies of scale,maximum production, there may be many orifices on the outer cylinder ofthe pastillator, typically at least 100, for example, between 100 and50,000, depending on the scale of operations. The pellets suitably maybe produced on the scale of 1 kg to 10 metric tons per hour, preferably1 to 10 metric tons per hour. For such operation, the pastillator wouldbe adapted for rotating at a rate which is sufficient to supply pelletsto the conveyor surface at the desired production rate.

Droplets or crystallizing pellets 18, formed by the pastillator 10, aredirectly received onto a moving surface 12 of a conveyor belt, which issubstantially level. By "substantially level" is meant not varying bymore than 10° from horizontal. By "moving surface" is meant any surfacewhich can support and transport the pellets. The moving surface 12generally moves relative to the pastillator, in a direction tangentialto the direction of rotation of the outer container of the pastillator.The moving surface 12 has a bottom surface 16 and a top surface 14, thelatter comprising the substantially level moving surface which supportsthe pellets. The moving surface 12 conveys the pellets through acrystallization section, which may also be referred to as a heatingsection. The moving surface is generally maintained at a constant speedfor passing the pellets through the crystallization section, althoughthe speed chosen can vary in order to vary the time the pellets arewithin the crystallization section.

A key feature or component of the present apparatus is thecrystallization section. The crystallization section begins at or verynear the point at which the pellets are received from the pastillator 10onto the moving surface and extends along at least a portion of theconveyor moving surface.

An important feature of the crystallization section of the apparatus isthat it includes means for controlling the temperature of the movingsurface, as it passes through the crystallization section, at anelevated temperature. Ovens containing a heating coil may be employed.In the preferred apparatus of the present invention, the temperature ofthe top surface 14 within the crystallization section is maintainedabove 50° C., depending on the surface material of the conveyor. If thesurface material is metal, then a conventional heater should be capable,in practice, of raising the temperature to at least 50° C., preferablyat least 100° C., more preferably between 100° C. and 225° C., which maydepend on the heat transfer coefficient of the surface. In the broadprocess of the invention, however, the temperature may vary below 50°C., if the conveyor surface has a lower heat transfer coefficient thanmetals such as steel.

The crystallization should be capable of maintaining a relatively steadytemperature, although some gradient along the crystallization section isallowable. Preferably, the temperature of the surface in thecrystallization section is carefully controlled, as further describedbelow.

Preferably, a portion of bottom surface of the moving surface 12 isheated within the crystallization section. It is also possible to have aheater prior to the point at which the pellets are received on theconveyor surface, in which case the crystallization section may onlyrequire insulation and/or slight heating. The crystallization sectionmay further comprise means for adjusting the temperature and/or flow ofa heat-exchange fluid and supplying a flow of the heat-exchange fluid tothe bottom surface 16, such as shown within the crystallization section20 in FIG. 1. In the embodiment shown in FIG. 1, an air heater 26supplies heated air to a lower plenum 24, enclosing a portion of thebottom surface 16 of the moving surface 12. The lower plenum 24generally contains an inlet and outlet for the heat-exchange fluid, sothe heat-exchange fluid can continuously circulate through the lowerplenum 24. The lower plenum 24 extends along the portion of the movingsurface 12 which comprises the crystallization section. In this way, thepellets 18 are subjected to proper heating immediately after beingformed and collected on the moving surface 12.

In order to obtain rapid heat transfer from the moving conveyor surfaceto the just-formed polymer pellets, it is preferred that the materialfor the conveyor moving surface 12 have a relatively high heat transfercoefficient. Metals are particularly useful for this purpose, especiallymetals, such as steel, with high heat transfer coefficients. Thus metalsare the preferred materials for the conveyor moving surface, althoughother materials, for example, plastic or plastic coatings are possible.

The temperature of the top surface 14 of the moving surface 12 withinthe crystallization section may be controlled automatically or manuallywith the use of a temperature sensor 28 located within thecrystallization section. Preferably, however, a temperature controllermay automatically control the temperature of the top surface 14 of themoving conveyor surface 12 in the crystallization section within apredetermined temperature range. Controlling the temperature, incombination with controlling the speed of the conveyor moving surfacesupporting the pellets, will result in the pellets 18 being subjected tothe predetermined temperature range for a minimum amount of time whichcan be predetermined. This occurs as the pellets 18 pass through thecrystallization section. Generally, the temperature controller comprisesa sensor 28 for determining the temperature of the top surface 14 withinthe crystallization section, a comparator (not shown) for comparing thetemperature determined by the sensor to a set point within thepredetermined temperature range, and a temperature adjustor (not shown)for adjusting the temperature of the heat-exchange fluid supplied to thebottom surface 16 of the moving surface 12. Conventional temperaturecontrollers are well known in the art, as will be appreciated by theskilled artisan, and are commercially available from a wide variety ofsources.

Controlling the temperature of the metal surface of the belt may attimes require the removal of heat from the heat exchange fluid or bottomsurface 16, i.e., relative cooling, although the crystallization sectionmay be heated relative to ambient. Typically, when a heat exchange fluidis supplied in a continuous flow to the bottom surface 16, and theset-point temperature is exceeded, a controller will typically signal noadditional heat input. This does not, however, contravene the spirit ofthe invention, since the general result is heating of the bottom surface16, and consequently the top surface 14.

In FIG. 1, a heater for the bottom surface 16 of the moving surface 12is within the crystallization section. The primary function of theheater is to heat the moving surface 12 such that the top surface 14 iswithin a predetermined temperature range. Heating the moving surface 12so that it is maintained at a temperature within the predeterminedtemperature range can be accomplished by a variety of means known tothose skilled in the art. Various embodiments and apparatus for heatingare encompassed within the scope of this invention.

In the preferred embodiment of FIG. 1, heating is primarily by means ofheating the bottom surface 16 of the moving surface 12. The overallsystem may also include additional, auxiliary heating means. Forexample, a second temperature-controlled (i.e., generally heated)heat-exchange fluid, preferably an inert gas to avoid degradation of thepellets 18, can be supplied to heat the portion of the top surface 14supporting the pellets that is within the crystallization section.Preferably the gas is inert. Suitable gases include nitrogen, the noblegases such as argon and helium, oxygen, air, and the like.

In this preferred embodiment, the pellets 18 are subjected totemperature control, at an elevated temperature, by means of both thehot moving surface 12 and from the flow of heated inert gas. The inertgas is preferably at a temperature less than that of the top surface 14.For example, for PET, the temperature of the inert gas, e.g., nitrogen,typically ranges from 25° C. up to 100° C., although higher temperaturesare feasible.

A flow of heated inert gas over the pellets may be provided in order tocontrol the temperature gradient that will exist through the thicknessof each pellet, thus serving to achieve more uniform crystallizationthroughout each pellet. The more uniform the temperature is throughoutthe pellet during the minimum predetermined amount of time, the moreuniform the crystallization will be within each pellet, althoughtemperature gradients within the pellets, to some extent, will likelyoccur while within the crystallization section. An important goal of thecrystallization section is to get the temperature of the polymer pelletsto the desired crystallization temperature as rapidly as possible and tomaintain it at a predetermined temperature for a minimum period of time.

As indicated above, while controlling the temperature of a continuousflow of inert gas, there may be temporary periods of time when the gasis not heated, in order that the set-point temperature is obtained. Theoverall effect, however, is to control the temperature, by means of thegas, the environment surrounding the just-formed pellets 18.

A second means for heating and supplying a continuous flow of a secondheat-exchange fluid is shown in FIG. 1 as a heater 22 for heating a flowof nitrogen supplied to an upper plenum 20. The upper plenum 20 canenclose the top surface 14 within the crystallization section, andgenerally contains an inlet and an outlet for continuously circulatingthe nitrogen through the upper plenum 20.

FIG. 2 is a cross-sectional view of an upper and lower plenum encasingthe crystallization section. As shown in FIG. 2, a conveyor belt 12covers the upper opening of the lower plenum, 24. The roller for thebelt is shown below by the dotted line. The conveyor belt 12 also servesto cover the bottom opening of upper plenum 20. Resting on the belt,seals 42, typically made of TEFLON® (DuPont, Wilmington, Del.), may beemployed to prevent excess loss of the heat-exchange fluid which iscirculated through the upper plenum 20.

As an example of auxiliary heating to assist in maintaining thetemperature of the top surface 14 within a predetermined range, a thirdheat-exchange fluid can be supplied to an internal chamber 34 located inthe upstream roller 30 of the conveyor. The internal chamber 34 mayinclude an inlet and outlet which are connected by conduits to a meansfor heating and circulating the third heat-exchange fluid. FIG. 1 alsoshows a heated pump 38, within a hot oil bath 43, for supplying thethird, heat-exchange fluid, e.g., a hot oil, through a conduit 36 to theinternal chamber 34 of the upstream roller 30. The roller is preferablyconstructed of a heat-conductable material to ensure that heat from theheated oil is efficiently conducted from the internal chamber 34,through the roller 30 to the bottom surface 16 of the conveyor belt.Heating the upstream roll 30 as described provides supplementary heatingwhich counteracts normal heat loss and lessens the burden on the primaryheater 26. It would also be possible, however, to provide primaryheating upsteam of the pellets, in combination with supplemental heatingand/or insulation following the point at which the pellets are receivedon the belt.

After the crystallization section, the now crystallized, low molecularweight pellets 18 can be collected and transported for furthertreatment.

The present apparatus can be used to make relatively robust and uniformpellets of a polyester polymer. One such process, which is particularlyadvantageous, will now be described.

In the preferred process, a polyester polymer in melt form having adesired intrinsic viscosity, IV is processed in an apparatus accordingto present invention. Generally, polymer having an IV ranging from about0.05 to about 0.40 dl/g is suitable. An IV ranging from about 0.09 toabout 0.36 dl/g is preferred.

The IV is determined as follows. A solvent is made by mixing one volumeof trifluoroacetic acid and three volumes of methylene chloride. PET, inthe amount of 0.050 g, is then weighed into a clean dry vial, and 10 mLof the solvent is added to it using a volumetric pipette. The vial isclosed (to prevent evaporation of the solvent) and shaken for 30 min oruntil the PET is dissolved. The solution is poured into the large tubeof a #50 Cannon-Fenske® viscometer, which is placed in a 25° C. waterbath and allowed to equilibrate to that temperature. The drop timesbetween the upper and lower marks are then measured in triplicate, andshould agree within 0.4 sec. A similar measurement is made in theviscometer for the solvent alone. The IV is then calculated by theequation: ##EQU1##

The present process can be integrated with a method of producing apolymer in melt form. Producing the polymer in melt form can beaccomplished in various ways, discussed above, and includes extrudingpolymer initially in the form of flake, pellets or chips. Additionally,an overall process can include polymerizing reactants in a reactor forpolymerization, for example, by melt polymerization, as discussed above.

In the preferred process, the polyester is initially at a firsttemperature which is at or above its melting temperature. For polyestersof interest, this initial temperature would be above 200° C. For PET,this initial temperature would be equal to or greater than about 250° C.It is preferred that the polymer melt is essentially amorphous, i.e.,less than about 5%, preferably less than 1% crystalline. If the polymermelt is not initially amorphous, and is instead semicrystalline, it isdesirable for the polymer to be thoroughly and uniformly heated aboveits melting temperature to ensure the semicrystalline areas aresufficiently melted.

The polyester polymers, at the above-mentioned first temperature, isformed into pellets in a droplet or pellet former, described above. Thepellets are collected, as they are formed, onto a substantially levelsurface which is maintained at a second temperature within acrystallization zone. (By substantially level is meant not more than 10°from horizontal). Pellets may be subjected to heating in thecrystallization zone, as described with respect to the apparatus of thisinvention, particularly if the belt is metal. The key feature of thecrystallization zone is that it allows temperature control of thejust-formed pellets, such that the pellets are subjected to theirdesired crystallization temperature immediately after they are formed.Accordingly, pellets may be produced which are robust and uniform, evenwhen involving low molecular weight polymer. Such pellets are suitablefor transport and further polymerization, for example, solid-statepolymerization.

In order to form polyester pellets suitable for transport and furtherprocessing, such as solid-state polymerization, the pellets should besubjected to contact with a conveyor surface at a temperature within apredetermined temperature range as rapidly as possible after formation.This predetermined temperature range for polyesters preferably rangesfrom about 80° C. to about 230° C., preferably about 110° C. to about190° C.

Additional preferred process embodiments are also described incocurrently filed commonly-assigned applications Ser. No. 08/375,873,now U.S. Pat. No. 5,540,868 and Ser. Nos. 376,600 and 08/376,596, nowabandoned and refiled as CIP Ser. No. 08/576,657, filed Dec. 21, 1995,respectively, all three applications hereby incorporated by reference intheir entirety.

Subjecting the just-formed polymer pellet to a surface temperaturewithin the predetermined temperature range will result in an immediatetemperature gradient between the polymer pellet, initially at or nearits melt temperature, and its surroundings. This should be done asquickly as possible in order to obtain the desired crystallinemorphology formed. The crystalline morphology is related to therobustness and abrasion resistance of the pellets, especially therobustness during later polymerization.

The pellets are maintained in contact with the hot surface for apredetermined amount of time, which for polyesters should be no lessthan about 3 seconds, preferably about 10 to 60 seconds. Generally, thetime needed to produce low molecular weight, crystalline polyesterpellets, having the desired crystallinity, will not exceed about severalminutes, although it would not be detrimental to maintain the pellets atthe desired temperature for longer periods of time, for example, 30minutes or more.

The term "crystalline" is herein defined to mean a crystallinity contentgreater than about 15%, preferably greater than 20%, and most preferablygreater than 30%, corresponding, respectively, for PET, for example, toa density greater than about 1.36 g/cc, preferably greater than about1.37 g/cc, most preferably greater than 1.39 g/ml. Thus, the termessentially-crystalline or crystalline, as used herein shall includewhat is commonly referred to as "semi-crystalline," as are mostpolyesters of interest. The amount of crystallinity can be determined byDSC (differential scan calorimetry). For example,essentially-crystalline PET is characterized by a total heat of fusion,expressed in J/g, of at least about 20, more preferably about 35, when140 J/g is used as the total heat of fusion of pure crystalline PET.Higher heats of fusion indicate more crystalline polymer. The percentcrystallinity within a sample of a polyester material or pellet can bedetermined by comparing the heat of fusion (J/g) of the crystallitespresent with the heat of fusion of the "pure" crystalline polyester.

The polyesters employed in the present invention or process comprisediacid or diester components, suitably including alkyl dicarboxylicacids which contain from 4 to 36 carbon atoms, diesters of alkyldicarboxylic acids which contain from 6 to 38 carbon atoms, aryldicarboxylic acids which contain from 8 to 20 carbon atoms, diesters ofaryl dicarboxylic acids which contain from 10 to 22 carbon atoms, alkylsubstituted aryl dicarboxylic acids which contain from 9 to 22 carbonatoms, or diesters of alkyl substituted aryl dicarboxylic acids whichcontain from 11 to 22 carbon atoms. The preferred alkyl dicarboxylicacids contains from 4 to 12 carbon atoms. Some representative examplesof such alkyl dicarboxylic acids include glutaric acid, adipic acid,pimelic acid and the like. The preferred diesters of alkyl dicarboxylicacids contain from 6 to 12 carbon atoms. A representative example ofsuch a diester of an alkyl dicarboxylic acid is azelaic acid. Thepreferred aryl dicarboxylic acids contain from 8 to 16 carbon atoms.Some representative examples of aryl dicarboxylic acids are terephthalicacid, isophthalic acid and orthophthalic acid. The preferred diesters ofaryl dicarboxylic acids contain from 10 to 18 carbon atoms. Somerepresentative examples of diesters of aryl dicarboxylic acids includediethyl terephthalate, diethyl isophthalate, diethyl or orthophthalate,dimethyl naphthalate, diethyl naphthalate and the like. The preferredalkyl substituted aryl dicarboxylic acids contain from 9 to 16 carbonatoms and the preferred diesters of alkyl substituted aryl dicarboxylicacids contain from 11 to 15 carbon atoms.

The diol component for polyesters used in the invention herein suitablycomprises glycols containing from 2 to 12 carbons atoms, glycol etherscontaining from 4 to 12 carbon atoms and polyether glycols having thestructural formula HO--(AO)_(n) H, wherein A is an alkylene groupcontaining from 2 to 6 carbon atoms and wherein n is an integer from 2to 400. Generally, such polyether glycols will have a molecular weightof about 400 to 4000.

Preferred glycols suitably contain from 2 to 8 carbon atoms, withpreferred glycol ethers containing from 4 to 8 carbon atoms. Somerepresentative examples of glycols, which may be employed as the diolcomponent of the polyester, include ethylene glycol, 1,3-propyleneglycol, 1,2-propylene glycol, 2,2-diethyl-1,3-propane diol,2,2-dimethyl-1,3-propane diol, 2-ethyl-2-butyl-1,3-propane diol,2-ethyl-2-isobutyl-1,3-propane diol, 1,3-butane diol, 1,4-butane diol,1,5-pentane diol, 1,6-hexane diol, 2,2,4-trimethyl-1,6-hexane diol,1,3-cyclohexane dimethanol, 1,4-cyclohexane dimethanol,2,2,4,4-tetramethyl-1,3-cyclobutane diol, and the like. Somerepresentative examples of polyether glycol (Polymeg®) and polyethyleneglycol (Carbowax®).

Branched or unbranched polyesters can also be used. The present processis applicable to both polyester homopolymers and polyester copolymersthereof. Further, the process of this invention is particularly usefulfor polyesters that do not crystallize easily, i.e., which requireheating, according to the present process, in order to crystallize. Thiswould include, for example, poly(ethylene terephthalate) (PET),poly(ethylene naphthalate) (PEN), poly(trimethylene terephthalate)(3G-T), and poly(trimethylene naphthalate) (3G-N). Generally, suchpolyesters having a glass transition temperature, T_(g), above about 25°C., and a melt temperature, T_(m), usually ranging from about 200° C. toabout 320° C.

Particularly preferred are polyesters modified with up to 10% by weightof a comonomer. Comonomers can include diethylene glycol (DEG),triethylene glycol, 1,4-cyclohexane dimethanol, isophthalic acid (IPA),2,6-naphthalene dicarboxylic acid, adipic acid and mixtures thereof.Preferred comonomers for PET include 0-5% by weight IPA and 0-3% byweight DEG.

As indicated above the crystalline polymer pellets produced according tothe present invention can be introduced into a solid-statepolymerization reactor for increasing the molecular weight of thepolymer. Preferably, the IV (intrinsic viscosity) of the polyester inthe pellets is below 0.4, preferably below 0.36, most preferably below0.3, and the IV of the polyester product of the solid-statepolymerization reactor is above 0.5, preferably 0.6 to 1.2. For example,for PET, the solid-state polymerization is suitably run at a temperaturebetween 200° and 270° C., preferably 220° and 250° C., provided it isbelow the melting point of the polymer for a period of time that ispreferably less than 24 hours.

EXAMPLE 1

This example illustrates a design for a demonstration unit. PET with anIV of 0.21 dl/g and COOH ends of 92.5 Eq/10⁶ g, which is produced by amelt-phase polymerization process is processed at 74 rpm through a twinscrew, 28 mm barrel extruder with six heated zones. The temperatures inthe zones are:

    ______________________________________                                        T1    T2        T3      T4      T5    T6                                      ______________________________________                                        130° C.                                                                      274° C.                                                                          285° C.                                                                        262° C.                                                                        284° C.                                                                      281° C.                          ______________________________________                                    

The discharge of the extruder is connected to a Zenith variable speedgear pump, the molten polymer material is pumped under pressure at aflow rate of 50 lbs/hr into a 60 cm (about two feet) wide ROTOFORMER®dropformer, manufactured by Sandvik Process Systems, Totowa, N.J. Theorifices, aligned in rows along the ROTOFORMER® are 1.5 mm in diameter.The feed temperature of the molten polymer material at the entry of theROTOFORMER® is about 285° C. The molten polymer material is dropformedin the form of droplets onto a conveyor 13. 8 ft in length, whichconsists of a continuously moving steel belt, which is also manufacturedby Sandvik Process Systems. The belt is heated by forced convection froman air blower which heats the bottom of the belt over approximately itsentire length to about 160° C. The molten polymer droplets aresolidified on the belt to provide uniform, hemispherical particles whichare conveyed to a collection bin. Based on experimental runs, in whichthe belt was not heated to an elevated temperature according to thepresent invention, it can be estimated that the head speed of thecylinder, the belt speed, and the average weight of the particles, ifproduced under the conditions described in this example, would be asfollows.

                  TABLE I                                                         ______________________________________                                        Example  Head Speed    Belt Speed                                                                             Avg. Particle                                 No.      (ft/min)      (ft/min) Weight (g)                                    ______________________________________                                        1        25.9          30       0.0369                                        2        33.3          30       0.0236                                        3        27.9          30       0.0221                                        4        87.6          60       0.0140                                        ______________________________________                                    

What is claimed is:
 1. A process for solid-state polymerization of apolyester polymer having a glass transition temperature T_(g) greaterthan about 25° C. comprising:(a) forming molten droplets by metering apolyester polymer melt through a plurality of outlets, each 0.5 to 5 mmin diameter, in a rotatable container; and (b) collecting the moltendroplets or crystallizing pellets, as they are formed, onto a movingsolid surface, adding or removing heat so that the surface is maintainedwithin a predetermined temperature range above 50° C. within acrystallization zone, such that the pellets are maintained in contactwith the surface within the crystallization zone for a predeterminedperiod of time; (c) introducing the pellets produced in step (b) into asolid-state polymerization reactor for increasing the molecular weightof the polyester polymer.
 2. The process of claim 1, wherein theintrinsic viscosity of the polyester polymer in the pellets is belowabout 0.36 and the intrinsic viscosity of the polyester product of thesolid-state polymerization reactor is above 0.5.
 3. The process of claim2, wherein the polyester polymer is PET.